The invention relates to an amplitude modulator circuit for modulating a video signal having periodically occurring reference signal portions on a carrier signal, the circuit comprising a modulator having a carrier input terminal for receiving the carrier signal, a modulation input terminal for receiving the video signal and an output terminal for supplying the amplitude-modulated signal obtained, the circuit also comprising a control loop which is coupled to the output terminal of the modulator and which includes a sampling circuit which is active during reference signal intervals, said control loop generating a direct current compensation signal to be added to the video signal.
Such a modulator circuit must be of such a construction that the amplitude-modulated signal at the output terminal does not control a carrier signal in the time intervals in which the video signal has a reference value. To this end the video signal is usually brought to a fixed level, for example by means of a clamping circuit, this level corresponding to an output signal which does not contain a carrier. Such a technique is used in color encoding circuits for the NTSC or the PAL standards. The amplitude of the color sub-carrier must then be zero during the reference signal intervals.
In practice a carrier signal may however yet occur at the output terminal of the modulator during these intervals, which is, for example, caused by an asymmetry in the modulator, a direct current offset in the clamping circuit or variations caused by fluctuations in the supply voltage and/or the temperature. In encoding circuits, such an unbalance of the output signal is known as "sub-carrier leakage". This leakage can be eliminated for the greater part by designing the modulator circuit such that a relatively large modulating signal drives the modulator, as a result of which the fault is comparatively small. However, this requires a large power. Another possibility is an accurate adjustment of various portions of the circuit but such an adjustment is time-consuming, while the result thereof may again be susceptible to an unwanted variation with time.
Swiss Patent Specification 282.896 discloses a circuit in which a control loop produces a direct current compensation signal as a result of which signal portions can be brought to a predetermined level. This level must, however, be adjusted and may vary with time.